[BUG] _Mismatch between header and data in `lcurve.out`

[thangckt]

Bug summary

It is seemly that nan values are not printed in lcurve.out My lcurve.out like this

#  step      rmse_val    rmse_trn    rmse_e_val  rmse_e_trn    rmse_f_val  rmse_f_trn         lr
# If there is no available reference data, rmse_*_{val,trn} will print nan
      0      8.14e+00      2.00e-01      2.57e-01    1.0e-03
   1000      3.23e-01      1.84e-03      1.59e-01    3.2e-06

DeePMD-kit Version

v3.0.0a1.dev146+g6c715d14

Backend and its version

Pytorch 2.3.0

How did you download the software?

Offline packages

Input Files, Running Commands, Error Log, etc.

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Steps to Reproduce

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Further Information, Files, and Links

No response

[njzjz]

The header is wrong when there's no validation data.